When you have a large number os posts it's impossible to create sitemaps by
using pseudocron or hooks like save_post. This plugin will
require a system cron schedule to visit a secret URL and trigger the sitemap
dump procedure.

It splits the sitemaps into 50,000 URLs each, and also tries to stabilize the
memory usage during the run, doing small queries and using a limited buffer
to write the files.

If you don't have administration privileges over your server, you can use a
cron job provider on the
Internet. If you do, please report your experience for the other users.